Earlier days, a cell containing a gas was used to analyze the molecular structure of many gases by transmitting electromagnetic waves of predetermined frequencies and detecting the reduction in the intensity of the electromagnetic waves at discrete frequencies after passage there through. The extent of microwave absorption by a gas at a particular frequency is designated as an absorption line. Each of the gas concentration has a unique set of absorption lines differing from that of any other gas, and the absorption lines being invariant to external factors such as time, pressure and temperature.
Besides using microwaves, RF waves or other waves are used to detect trace gases. Instruments are used to measure carbon cycle gases in the atmospheric column which need comparable ground validation measurements. Currently, measuring carbon dioxide (CO2) and methane (CH4) in the atmospheric column is by a network which is based on ground-based Fourier Transform Spectrometers that record direct solar spectra in the near-infrared spectral region. For the spectra, accurate and precise column-averaged abundance of CO2, CO, C2H2 and CH4 are retrieved. Some globally distributed sensing network can also be used to trace gas concentration.
In the fields of metrology and sensing applications, lasers are used to provide a source for optical signals and to process at various wavelengths. Sensing applications use tunable lasers to monitor absorption peaks in gases or liquids in order to identify these elements and to measure their concentrations. Laser heterodyne radiometry is a technique for detecting absorption signals that was adapted from radio receiver technology and optical communication. In a radio receiver, a weak input signal from a radio antenna is mixed with a stronger local oscillator signal. Since the early nineteen seventies, laser heterodyne radiometry (LHR) has been used for atmospheric studies. The beat signal of the absorption region and the local oscillator is detected with an optical receiver and then the RF signal is amplified using a set of video signal amplifiers, bias-tee, filters, and a chain of amplifiers. The quality of the recovered signal is greatly dependent on the noise performance of the amplifier chain that produce unpredictable results that suffer from repeatability of the magnitude of the signal changes, specifically for small signal changes that are impossible to monitor with such technique. In addition to those issues, demand frequent calibration cycles and changes in the ambient temperature affects the RF amplifier response.
The tunable diode laser absorption spectroscopy in a process industry became one of the most promising techniques for online trace gas analyzing. Restricted by its principle, the measurement result of tunable diode laser absorption spectroscopy system is seriously affected by temperature and gas pressure variation. For this reason, most tunable diode laser absorption spectroscopy systems employ temperature and pressure sensors, which can provide information for partly correcting the error.
Prior art systems use analysis system which comprises an optical module, a fluid module, an electrical module, and a mechanical packaging module to trace gas which is complex, high cost, and less reliable.
Hence, there exists a need in the art for a method and system comprising a tunable laser with pseudorandom Binary sequence generator to accurately estimate the strength and changes of the monitoring signal for a sensing application.